I'm sure I'll make someone mad by saying this, but the vendor that allows you to pick your own blend is sacrificing product quality in an effort to make everybody happy. Juice does not taste the same in VG as it does in PG. If you design your recipe to work at 70PG/30VG, or 100%VG, then when you make that recipe in other ratios it's just not the same juice. I guarantee you that almost none of the vendors that offer unlimited PG/VG choices are actually taking the time to adjust their recipes for different ratios. So what you end up with is a juice designed for say 70PG/30VG that tastes bland and gutless in all-VG. There are exceptions to this, but for the most part a vendor that lets the customer pick their own recipe is confused. When I see this all it tells me is that they don't care or they don't have a lot of confidence in their juice. OR all they're doing is squirting some DIY flavors in base, which is what you could be doing at home if you wanted to, and then it really doesn't matter anyway.
A perfect recipe is a work of craft and skill. I don't want to change anything about a good wine or a small farm cheese, and I don't want to change anything about my juice. I want to feel like I am buying juice that offers something more than I could make at home.
I do feel that all vendors should offer unlimited options for customers that have specific preferences, but I also think that it should be per request. This is my opinion and not a condemnation of your favorite vendor or preferences in juice.